Gas turbines are widely used in commercial operations for power generation. A typical gas turbine includes a compressor, one or more combustors downstream from the compressor, and a turbine downstream from the one or more combustors. A working fluid such as air flows through an inlet of the compressor wherein the compressor imparts kinetic energy to the working fluid to bring it to a highly energized state. The compressed working fluid exits the compressor and flows to the combustors. The combustors mix fuel with a first portion of the compressed working fluid, and the mixture of fuel and working fluid ignites to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases flow to the turbine where they expand to produce work. A second portion of the compressed working fluid may be used to cool various components within the gas turbine.
It is widely known that the thermodynamic efficiency of a gas turbine increases as the operating temperature, namely the combustion gas temperature, increases. Higher temperature combustion gases contain more energy and produce more work as the combustion gases expand in the turbine. However, higher temperature combustion gases may produce excessive temperatures in the turbine that can approach or exceed the melting temperature of various turbine components.
A variety of techniques exist to allow the combustors to operate at higher temperatures. For example, air may be extracted from the compressor, bypassed around the combustors, and injected directly into the stream of combustion gases in the turbine to provide conductive and/or convective cooling to the turbine stages. However, the air extracted from the compressor has already been compressed, and thus heated, by some amount, thereby reducing the heat removal capability of the extracted air. In addition, since the extracted air bypasses the combustors, extracting air from the compressor reduces the volume of combustion gases and overall efficiency and output of the gas turbine.
Another method to cool turbine components may include circulating a portion of the compressed working fluid through various flow paths within the gas turbine. For example, the turbine typically includes stationary nozzles (stators) and rotating blades (buckets). The stators and/or buckets may include internal passages through which cooling air may flow. As the cooling air flows through the internal passages, the cooling air directly contacts the walls of the internal passages to remove heat from the stators and/or buckets through conductive or convective cooling. However, the elevated temperature of the compressed working fluid available for cooling generally limits the rate of heat transfer between the compressed working fluid and the walls of the internal passages. Other methods for cooling the gas turbine may include directing a cooling fluid, such as steam into various portions of the gas turbine. However, these methods may create problems with oxidization within the gas turbine and may reduce overall plant efficiency. Therefore, a closed loop cooling system that can remove heat from the compressed working fluid flowing though the gas turbine would be useful.